Slip casting

ABSTRACT

FINE METAL COMPOUND POWDERS CAPABLE OF BEING REDUCED TO THE METALLIC STATE ARE MADE INTO A CASTING SLIP,CAST INTO A SLIP CASTING MOLD TO FORM A SLIP CAST OBJECT WHICH IS REMOVED FROM THE MOLD, REDUCED AND SINTERED INTO A PORE-FREE HIGH DENSITY OBJECT.

June 27, 1972 J, R. SAGMULLER ETAL 3,672,882

SLIP CASTING Filed May 26. 1969 vm 4\ l4 LIIANI Fig. l

Fig. 20 2b United States Patent 3,672,882 SLIP CASTING Joseph R. Sagmuller and Richard 1. Hunter, Columbus,

Ohio, assiguors to The Battelle Development Corporation, Columbus, Ohio Filed May 26, 1969, Ser. No. 827,846

Int. Cl. B22f 1/00 US. Cl. 75-211 Claims ABSTRACT OF THE DISCLOSURE Fine metal compound powders capable of being reduced to the metallic state are made into a casting slip, cast into a slip casting mold to form a slip cast object which is removed from the mold, reduced and sintered into a pore-free high density object.

BACKGROUND This invention relates to improvements in slip casting and relates in particular to a new and novel method for making slip cast and sintered metal articles.

Slip casting is a method for making articles by introducing a slurry or dispersion of a particulate material in a carrier liquid into a mold that is constructed of a substance that is disposed to absorb the carrier liquid. The mold draws olf the carrier liquid or vehicle leaving the particulate material deposited on the inner walls of the mold in the desired shape. Partial or complete drying causes the slip-cast deposited particulate material to shrink a sufficient amount for removal from the mold. The slipcast article consisting of compacted particles is generally further consolidated into a solid body usually by heat treatment and fusion of the particles.

The process is very old as it applies to the casting of clay articles, but is relatively new as it applies to the casting of metal and metal oxide particles. Whereas slipcast clay articles are fired to effect ceramic bodies, slipcast metals and metal oxides are sintered to effect dense metal or ceramic objects.

The art of slip casting, particularly as it relates to the casting of metal oxide and metal powder articles, is very complex. The slip must be of a viscosity that lends itself to easy pouring since if the slip is too thick it will not fill in the details of the mold. If the dispersed material settles out too rapidly when making hollow castings the wall thickness of the castings will vary. The flow properties or viscosity of a slip must be reasonably constant over a range of solid-liquid ratios. There must be some shrinkage to permit the casting to be removed from the mold but excessive shrinkage may cause strains in the walls of the greenware and increases the chances for cracking both before and during firing or sintering. Additional factors to consider in formulating a slip include greenware strength, the release rate from the molds and the tendency of slips to undergo chemical change during storage.

The parameters involved in creating a slip that will meet the above-enumerated varying properties include the selection of the carrier liquid or vehicle, additives to the slip such as dispersing agents and the particle size of the oxide or metal.

Economic considerations will generally dictate that the carrier liquid or vehicle be water and the preferred dispersing agent, if required at all, will vary in accordance with the casting being made and exact slip-casting practice involved. In any event, regardless of these considerations, the single most important parameter in effecting proper rheological control over the slip is particle size and particle size distribution.

Generally, slip casting of metals is carried out utilizing particles below 10 microns in diameter. Such particles or powders are not, however, classified so that they actually consist of powders or particles having a particle size distribution ranging from about 20 to 1 microns. If the particles are finer the exercise of proper rheological control over the' slip becomes difficult. Fine particles tend to interact with the surrounding atmosphere so that minor changes in the ionic nature of the atmosphere can effect undesirable viscosity changes.

Considerations relating to the slip casting of metal particles include the pyrophoric nature of powdered metals. For example, iron powder may be manufactured by the direct reduction of iron oxide powder. However, where the iron powder produced has a particle size distribution wherein a major portion is composed of particles of less than about 2.5 microns diameter the powder reverts to iron oxide within a few minutes of being exposed to air.

Although this problem can be circumvented to a large extent by utilizing controlled environments and by limited oxidation pretreatment, it is a definite limiting factor on particle size when slip casting metal powders.

The ultimate density of sintered metal compacts is related to the size of the metal particles. Generally, the finer the particle size the greater the density. As shown above, however, the pyrophoric nature of metal powders such as iron powders has a limiting effect on the usable particle sizes requiring pretreatment or controlled atmosphere processing. However, where powdered metals are slip cast an added limitation on the size of particles which may be utilized relates to the rheological control over the slip.

A recent development in creating metal objects through slip casting techniques involves slip casting of metal oxides, reducing the cast metal oxide object by heat treatment in a reducing atmosphere and sintering (M. Zadrovitch and A. Mitav Mohanty, Powder Metallurgy, 1965, volume 8, No. 15, pages 152-161) to obtain a metal object. The products obtained by this procedure exhibit a considerable degree of porosity. Additionally the original particles are visible in the microstructure and the density of these products are approximately only percent of a theoretically fully dense product. Such porosity and low density properties renders the process commercially unattractive.

THE INVENTION We have discovered that metal compound powders may be slip cast, reduced, and sintered to obtain metal objects having relatively smooth pore free surfaces and density properties exceeding percent of theoretically percent dense cast metal if at least 35 percent, by weight, of the oxide particles have a particle size of 10 microns or less as determined by Coulter Counter Analysis. Preferably, the particle size distribution will be considerably below the maximum of 35 percent, by weight under 10 microns and will have a mean particle size no greater than about 6 microns and at least 25 percent, by weight, of the particles will be below 2.5 microns in diameter. The minimum particle size is that which can be tolerated in creating a slip with acceptable viscosity for slip casting. Optimum densities are obtained with powders that have a particle size distribution wherein substantially all of the particles are below 10 microns in size and at least 50 percent, by weight, of the particles are under 1 micron in diameter.

We have found that it is possible to make a castable slip from metal oxide powders such as iron oxide powders having a particle size distribution within the above recited ranges and slip cast objects which, when reduced and sintered, present a nonporous structure that exhibits densities in excess of 90 percent of theoretical.

We have also found that objects slip cast, reduced, and sintered in accordance with the method of the present invention exhibit superior reproducibility of the' mold surface when compared to prior practices. It is possible to take advantage of the shrinking characteristics of reduced and sintered slip cast metal compounds to provide particularly fine detail on the surface of the cast object.

We further find that where the particle size distribution meets the above recited parameters reducing and sintering times combined are usually less than the time and/or temperatures ordinarily required for sintering powdered metal objects.

THE DRAWINGS FIG. 1 is an illustrative cross-sectional view of a composite slip cast shape that constitutes an embodiment of the present invention.

FIGS. 2a and 2b are illustrative fragmented enlarged cross-sectional views of a mold surface and corresponding slip cast surface showing the slip cast surface before reduction and sintering and after reduction and sintering respectively.

DETAILED DESCRIPTION The method of the present invention is applicable to any reducible metal compound particularly those susceptible to reduction with hydrogen which have standard free energies of reaction with hydrogen that are less than about kilocalories per gram atom of hydrogen at the reduction temperature. The metal compounds of particular interest are the metal oxides such as the oxides of Fe, Co, Ni, Cu, Mo, and W.

Although the use of hydrogen to provide the environment for reducing the metal compound powders to elemental metal is a preferred embodiment of the present invention we have found that other reducing materials may be employed. For example, we have found that the above-recited metal compounds and particularly iron oxide can be reduced by partially or wholly substituting carbon monoxide for the hydrogen reducing environment.

Any metal compound powders having particles of any general shape (i.e., spherical, oblong, needles, or rods, etc.) and originating from any source (i.e., ore deposits, ore concentrates, precipitates, etc.) may be employed in the creation of a slip for casting, reducing, and sintering in accordance with the present invention. The sintered article derived will possess a substantially pore free structure, a smooth surface, and will exhibit densities in excess of 90 percent of theoretically completely dense material. We have, however, discovered that metal oxide powders obtained by the process of spray drying provide superior slips that reduce and sinter in a manner to provide objects of greater density and better surface and structural integrity than slips made from other sources of metal oxide.

Spray drying of solutions containing soluble metal compounds to effect metal oxide powders is a well known prior art procedure. For example, this method is utilized to regenerate hydrochloric acid pickling solutions that have been used in the iron and steel industry to remove mill scale and other forms of iron oxide from iron and steel products. The used aqueous pickling solution containing up to about 11 percent, by weight, free hydrochloric acid, and up to about 35 percent ferrous chloride is sprayed through a nozzle into a heated chamber (about 1000 F.) where the ferrous chloride is converted into iron oxide and hydrochloric acid, as follows:

One version of the process is described in the article Liquor Regeneration Slashes Cost of Steel Pickling, by Joseph A. Buckley, Chemical Engineering, Jan. 2, 1967, pages 56-58.

Regardless of the exact parameters or specific apparatus used, spray-dried metal oxides, and particularly spraydried iron oxides are believed to consist of minute hollow spheroids. The spheriods themselves cannot be used to make satisfactory slips for slip casting, reducing, and sintering in accordance with the method of the present invention and it is our theory that when fragmented the resultant powders produce a slip of superior characteristics for use in conjunction with the method of the present invention.

We believe that the fragmented spray-dried particles tend to agglomerate making accurate particle size determinations difficult. However, Coulter Counter measurements indicate that after three hours of dry ball milling in our mill the powder is substantially all under one micron size (average diameter).

We believe that the tendency of the fragmented spraydried metal oxide particles to agglomerate, settle, and interlock or pack more closely when slip cast than ordinary slip cast powders yet providing enough porosity to let the carrier liquid drain out of the Slip and into the mold is responsible for the superior product. The result is a slip cast article of high green density and a structure after reducing and sintering that at least approximates metals cast from the molten state.

By the term fragmented as it relates to the hollow spheroidal particles obtained by the above described spray drying technique, we mean the breaking up of the hollow spheroids into smaller particles. Such breaking up is most conveniently accomplished by mechanical means such as grinding. We have had particular success in ball milling such spheroidal particles for periods of from about 1 to 10 hours; however, other grinding techniques may be employed.

The slip cast metal compounds of the present invention may, of course, consist of blends or mixtures of two or more compounds of varying metals so as to effect a metal alloy product. Similarly, the metal compound particles or a portion thereof may consist of bi or multi metallic compounds containing more than one metal. Also, elemental metals or metal alloy powders may be blended with the metal compound slip. In the latter practice the advantages of the present invention are largely lost where more than about 50 percent, by volume, of the mixture consists of metal particles.

When practicing the preferred embodiment of the present invention wherein spray-dried and fragmented metal oxides are utilized to produce the slip, it will be preferred that the alloying compounds also be of the spray-dried-fragmented variety. Some advantage will be experienced in utilizing any amount of spray-dried and fragmented metal oxides in the slip regardless of how small the proportion of these metal compound fragments are in relation to the metal compound particles; however, such advantages (green and sintered densities and sintered structure) are not readily discernble where such fragments do not constitute at least about 10 percent, by volume, of the particles present.

We have found that green slip cast metal oxide objects cast in accordance with the present invention may be joined by positioning such objects in abutting relationship to one another prior to reducing and sintering. After reducing and sintering, the weld or juction between the abutting objects is not discernible by microscopic examination of polished and etched segments. This discovery is significant since it increases the versatility of the process immeasurably. By following this practice it is possible to join solid and hollow slip cast objects that cannot be slip cast in a single mold. Additionally, it is possible to slip cast around an already green slip cast part to effect unique cast structures.

Preferably where joining two green slip cast objects they should be positioned adjacent one another while still damp or before being completely dried. We have had particular success by painting casting slip onto the surfaces that are to abut prior to reducing and sintering.

The advantages of slip casting in accordance with the method of the present invention as compared to conventional metal casting techniques are considerable. The greatest advantage relates to the reproducibility of the mold surface, particularly in terms of detail in the final product. Where metal compounds such as metal oxides are slip cast, reduced, and sintered, shrinkage from the as-cast to the as-sintered object is considerable. For example, iron objects derived from reducing and sintering slip cast iron oxide are about two-third of the size of the original slip cast objects. However, where metal particles are slip cast and sintered shrinkage is generally less than percent and in ordinary molten metal casting shrinkage is less than 2 percent. The sintered objects of the present invention exhibit relatively smooth, bright, and continuous surfaces and additionally exhibit remarkably accurate reproductions of the mold surface though reduced in size to about two-thirds of the original slip casting. This is significant in that it enables one to provide tolerances in surface detail not previously regarded to be feasible.

The above phenomenon is best illustrated by FIGS. 2a and 2b of the drawings.

To attain the slots 16 of width W in a metal object 18 (FIG. 2b) it is necessary to provide a projection 20 on the inner surfaces of the mold 22. Where the slot 16 consists of fine detail, for example Where the width W is to be about 10 mils a groove of about 10.1 mils must be provided on the surface of the mold pattern where one is casting molten metal and about 10.5 mils where one is slip casting and sintering metal particles (the fraction .1 and .5 accounting for shrinkage). Such a small groove is difiicult to produce by any conventional means such as machining with any degree of accuracy. Further, conventional molten metal casting and slip casting techniques will not consistently provide accurate detailed surface reproductions of depressions as small as 10 mil wide grooves. When employing the method of the present invention, the mold pattern groove and mold projection 22 will have a width (W-l) of about mils since after reducing and sintering the casting including groove 16 will shrink to two-thirds of its orginal size to provide an accurate reproduction of the pattern groove but having a width (W) of the desired 10 mil dimension. It is obviously easier to provide a 15 mil groove in the mold surface than a 10.1 or 10.5 mil groove; consequently, the method of the present invention constitutes a considerable advance in the art of casting metal objects where surface details are concerned.

EXAMPLES Pickle-liquor iron oxide powders (Fe O hollow spheroids from spray dried aqueous solutions of FeCl were dry ball milled for three hours. Coulter Counter measurements of such fragmented powders show an average particle size of about 0.80 micron, the largest particles being about 10 microns. This powder was used in making up the following casting slip:

600 grams iron oxide 1.8 ml. Darvan 1 .3 gram citric acid Balance, water 1 A water-soluble dispersing agent made by the It. '1. "antlerbilt Company.

- pH values of 1.5 to 7 have been employed.

The slip was cast into plaster of Paris molds having mold cavities in the shape of a horsehead bookend and an ornamental cup (both about 2 inches in diameter and 4 inches deep). The wall thickness of these objects was about 0.25 inch.

The cast slips were permitted to dry sufiiciently to shrink away from the mold cavity and were then removed from the molds and dried in a drying oven at 140 F. They were then heated in the presence of a hydrogen atmosphere at approximately 1200 F. for a sufiicient time to reduce the iron oxide to elemental iron and were then sintered at about 2100" F. in a reducing atmosphere. The resultant objects exhibited crack-free smooth finishes and densities in excess of percent of theoretical. Although they had shrunk to approximately one-third of their original size they retained the original ornamental shape.

An iron oxide slip of the aforementioned composition and characteristics (spray dried Fe O spheroids dry ball milled for three hours) was cast into a plaster of Paris mold to form a closed end hollow cylinder (1% inches OD. and 1% inches long). This part is identified as part 12 in the accompanying drawing. The cylinder 12 was removed from its mold, and while still moist, was placed upside down in a larger cup-shaped plaster of Paris mold. The larger mold was then filled with slip forming the cup (about 5 inches ID. at the lip of the cup) shaped member 14 of the figure of the drawings. The as-cast base portion of the cup shaped member was in contact with the hollow cylinder (see the accompanying drawing) so as to form a single object consisting of the hollow cylinder and cup shaped member.

After removal from the second mold the assembly was dried, reduced, and sintered as follows:

(a) dried at F.

(b) heated to 1150 F. at 300 F. per hour in a hydrogen atmosphere,

(c) held at 1150 F. for /2 hour in a hydrogen atmosphere,

(d) heated from 1150 F. to 2150 F. at 300 F. per

hour in a hydrogen atmosphere,

(e) held at 2150 F. for /2 hour in a hydrogen atmosphere, and

(f) furnace cooled.

After reduction and sintering, the shape was one piece and exhibited a bright, crack-free surface. A photomicrograph at area A as shown by the accompanying drawing showed a fine grained iron structure (ASTM grain size of 8 or less). No bond line was discernible. Density as determined by loss of weight in water Was about 93 percent of the density of iron.

Accurate particle size determinations of fine grained powders are diflicult to obtain, particularly where the particle size distribution of such powders includes a fraction that is less than 10 microns in diameter. Such determinations are most difficult where the particles are of nonuniform shape. For example, if the particles consist of crushed or ground spheroids as is speculated in regard to ball milled spray dried HCl pickle liquor oxides many of the particles are likely to be of a relatively elongated or semicircular shape (sections of a hollow spheroid) so that it is diflicult to determine actual diameter. Elongated particles will not pass through a screen having a mesh that is designed to accommodate a relatively symmetrically shaped particle of equivalent mass. As a result particle size and particle size distribution measurements vary to a considerable degree for a given powder between the known methods and procedures for making such determinations. For the purposes of the present specification and claims we have used Coulter Counter Analysis to make particle size determinations. In this system the particles are suspended in an electrically conductive liquid and drawn through a small orifice. An electric flow is induced through the orifice by two immersed electrodes, one on each side of the orifice. As the particles flow through the orifice the change of electrical resistance between the electrodes is measured to determine particle size. Thus, the measure is one based on particle mass and is not affected by shape.

For the purposes of the present specification and the claims all particle size determinations and limitations are in terms of Coulter Counter measurements and shall include metal compound particles meeting such determinations irrespective of particle size determinations by other means.

We claim:

1. The method of making high density sintered metal objects comprising:

(a) providing a casting slip composed of particulate metal compounds wherein said particulate metal compounds are selected from the group consisting of the oxides of iron, cobalt, nickel, copper, molybdenum, and tungsten mixed with a carrier liquid, at least 35 percent, by weight, of the metal compound particles being less than 10 microns in diameter;

(b) casting said slip in a liquid absorbent mold;

(c) removing the cast object from said mold;

(d) exposing said cast object to a gaseous reducing environment disposed to convert said metal compound particles to metal particles; and

(e) exposing said cast object to a temperature disposed to effect sintering of the metal particles so as to increase the density of said object.

2. The method of claim 1 wherein said metal compound particles have a mean particle size no greater than 6 microns and at least 25 percent, by weight, of which do not exceed 2.5 microns.

3. The method of claim 2 wherein at least 50 percent, by weight, of the particles are under 1 micron in diameter.

4. The method of claim 2 wherein said metal compound particles consist essentially of particles derived from spray drying solutions of soluble compounds of a metal of the group consisting of iron, cobalt, nickel, molybdenum and tungsten.

5. The method of claim 3 wherein said compounds of a metal of the group consisting of iron, cobalt, nickel, molybdenum and tungsten particles are derived from spray drying solutions of soluble metal compounds.

6. The method of claim 2 wherein said slip consists of at least 50 percent, by volume, particulate metal compounds balance essentially metal particles.

7. The method of claim 6 wherein said slip consists of at least 50 percent, by volume, of comminuted spray dried metal oxides.

8. A method for making high density sintered metal objects comprising:

(a) providing at least two casting slips composed of particulate metal compounds wherein said particulate metal compounds are selected from the group consisting of the oxides of iron, cobalt, nickel, copper, molybdenum, and tungsten mixed with carrier liquids, at least 35 percent, by weight, of the metal compound particles being less than 10 microns in diameter;

(b) casting each said slip into individual liquid absorbent molds;

(c) removing the individually cast objects from said molds;

(d) positioning said cast objects into abutting relationship with one another and exposing said objects to a gaseous reducing environment disposed to convert said metal compound particles to metal particles; and

(e) exposing said abutting cast objects to a temperature disposed to effect sintering of the metal particles so as to increase the density of said objects and effect a metallurgical bond therebetween.

9. The method of claim 8 wherein at least one of the abutting surfaces of said objects to be bonded is coated with at least one of said slips prior to their being placed in an abutting relationship in preparation for the steps of reducing and sintering.

10. A method for making high density sintered metal objects comprising:

(a) slip casting at least one object in an absorbent mold from a slip composed of particulate metal compounds wherein said particulate metal compounds are selected from the group consisting of the oxides of iron, cobalt, nickel, copper, molybdenum, and tungsten mixed with carrier liquids, at least 35 percent, by weight, of the metal compound particles being less than 10 microns in diameter;

(b) transferring said slip cast object to a second absorbent mold and casting a second slip into said second mold to form a second slip cast object that contacts and abuts at least a portion of said object, said second slip being composed of particulate metal compounds mixed with a carrier liquid, at least 35 percent, by weight, of the metal compounds of said second slip being less than 10 microns in diameter;

(c) removing said abutting slip cast objects from said second mold and exposing said objects to a gaseous reducing environment disposed to convert said metal compound particles to metal particles; and

(d) exposing said abutting cast objects to a temperature disposed to effect sintering of the metal particles so as to increase the density of said objects and effect a metallurgical bond therebetween.

References Cited UNITED STATES PATENTS 2,979,401 4/1961 Szymaszek -223 3,489,555 1/1970 Thellmann 75-211 3,510,292 5/1970 Hardy et a1. 75--206 3,322,536 5/1967 Stoddard et a1.

FOREIGN PATENTS 468,518 7/1937 Great Britain 75222 OTHER REFERENCES Zadrovitch et al., Powder Metallurgy, 1965, vol. 8, No. 15, pp. 152-160.

CARL D. QUARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner 

